The present application relates to a communication device performing a communication operation as a reader/writer (initiator) transmitting a request command or as a transponder (target) returning a response command in response to a request command in non-contact communication, and also relates to an antenna device for use in non-contact communication. In particular, the present application relates to a communication device, antenna device, and communication system performing non-contact communication with loop-antenna electromagnetic induction.
As a communication system in which a communication terminal without its own electric-wave generating source wirelessly transmits data to a communication counterpart, a non-contact communication system, called a radio frequency identification (RFID) system, is applied to many non-contact IC cards. The RFID system includes an integrated circuit (IC) card as a transponder and a device to read and write information from and to the IC card (referred to below as a reader/writer). The reader/writer starts intercommunication by initially outputting an electromagnetic wave (that is, taking the initiative in communication), so it is also called an initiator. The transponder, such as an IC card, is a target returning a response (intercommunication start response) in response to a command (intercommunication start request) from the initiator.
Examples of types of non-contact communication techniques applicable to RFID include an electrostatic coupling type, an electromagnetic induction type, and an electric-wave communication type. Also, according to the transmission distance, RFID systems can be classified into three types, that is, a close-coupled type (0 to 2 mm or shorter), a proximity type (0 to 10 cm or shorter), and a vicinity type (0 to 70 cm or shorter), and are defined by international standards of, for example, ISO/IEC 15693, ISO/IEC 14443, and ISO/IEC 15693, respectively. Among these, proximity-type IC-card standards complying with ISO/IEC 14443 include Type A, Type B, and FeliCa®, for example.
Furthermore, near field communication (NFC) developed by Sony Corporation and Koninklijke Philips Electronics N.V. is an RFID standard mainly defining specifications of an NFC communication device (reader/writer) communicable with an IC card of each of Type A and FeliCa, and became an international standard as ISO/IEC IS 18092 in December, 2003. The NFC communication technique inherits Felica of Sony Corporation and Mifare of Koninklijke Philips Electronics N.V., which are widely applied to non-contact IC cards; with the use of a band of 13.56 MHz, non-contact bidirectional communication of a proximity type (on the order of 10 cm) can be performed through electromagnetic induction (NFC defines not only communications between a card and a reader/writer but also active-type communications between readers/writers).
Non-contact communication in the past is mainly for billing and personal authentication, and communication rates of 106 kbps to 424 kbps are sufficient. However, in consideration of various applications, such as streaming transmission, to exchange large-capacity data with the same access time as before, the communication rate is increased. For example, in FeliCa communication, a multiple of 212 kbps, such as 424 kbps, 848 kbps, 1.7 Mbps, or 3.4 Mbps, is defined, and 212 kbps or 424 kbps is mainly used now. In the future, the communication rate may be increased to 848 kbps, 1.7 Mbps, or 3.4 Mbps.
FIG. 16 illustrates the basic configuration of an NFC communication system. The NFC communication system includes an initiator that starts communication and a target as a communication target.
Specifically, the initiator is an NFC-compliant reader/writer (R/W) that operates in a reader/writer mode. The reader/writer as the initiator is connected to a host device via a host interface, such as a universal asynchronous receiver-transmitter (UART). The host device corresponds to a personal computer (PC) and an embedded central processing unit (CPU) inside the reader/writer. The target is a transponder, such as an NFC-compliant card, or an NFC-compliant reader/writer that operates in a card mode (these examples of the target are also collectively referred to below simply as a card). The card may be stand-alone, or may be connected to the host device.
In passive-type intercommunication, the initiator performs amplitude shift keying (ASK) modulation on a carrier signal at 13.56 MHz emitted from it to superpose transmission data for transmission to the target. The target performs load modulation on a non-modulated carrier at 13.56 MHz sent from the initiator to transmit the transmission data to the initiator. In active-type intercommunication, the initiator and the target each perform ASK modulation on a carrier signal at 13.56 MHz emitted from them to superpose transmission data for transmission to its communication counterpart.
Upon receiving a communication start command from the host device (refer to (1) in FIG. 16), the initiator first sends a carrier wave. Then, to confirm whether the target is present in a communicable space, the initiator transmits a response request signal through a technique defined in the standard (about the carrier frequency, data modulation speed, and data details) (refer to (2) in FIG. 16).
By contrast, the target is first started by being supplied with power by an induced electromotive force of the carrier sent by the initiator, and enters a receivable state after which the target receives a response request signal sent from the initiator. Then, when the received response request signal is a signal matching its own type, the target performs load modulation on the non-modulated carrier from the initiator through the technique defined by the standard (about the data modulation speed, response timing, and data details) to make a response with a response signal including its own identification information (refer to (3) in FIG. 16).
Upon receiving the response signal from the target, the initiator transmits that information to the host device (refer to (4) in FIG. 16). Upon identifying the number of targets present in the communicable space and identification information of each target, the host device makes a transition to a phase of communication with a specific target according to an operation program (firmware). With this, passive-type intercommunication is established. After the communication is established, the initiator typically continues to output a carrier wave until necessary communication ends so as to send necessary power to the target.
As with the response request operation described above, also at the time of data communication, data transmission is performed through intensity modulation of the carrier wave from the initiator to the target and load modulation of the non-modulated carrier from the target to the initiator.
FIG. 17 mainly illustrates an example of the structure of an inductive-coupling portion of a non-contact communication system of an electromagnetic induction type. With electromagnetic coupling between antenna resonant circuits 12 and 32 included in an initiator 10 and a target 30, respectively, information signals are transmitted and received.
The antenna resonant circuit 12 of the initiator 10 includes a resistor R1, a capacitor C1, and a coil L1 as a loop antenna, and transmits an information signal generated by a processing unit 11 to the target 30. The antenna resonant circuit 12 also receives an information signal from the target 30 for supply to the processing unit 11. Here, the resonance frequency unique to the antenna resonant circuit 12 is set at a predetermined value in advance based on a capacitance of the capacitor C1 and an inductance of the coil L1. The coil L1 as a loop antenna is magnetically coupled to a coil L2 of the target 30, which will be described further below, with a coupling coefficient K13.
On the other hand, the antenna resonant circuit 32 of the target 30 includes a resistor R2, a capacitor C2, and a coil L2 as a loop antenna, and transmits an information signal generated by a processing unit 31 and modulated by a load-switching modulation circuit 33 to the antenna (coil L2) of the reader/writer (initiator) 10. The antenna resonant circuit 32 also receives an information signal from the reader/writer for supply to the processing unit 31. Here, the resonance frequency of the antenna resonant circuit 32 is set at a predetermined value in advance based on a capacitance of the capacitor C2 and an inductance of the coil L2. The coil L2 as a loop antenna is magnetically coupled to the coil L1 of the initiator 10 described above with the coupling coefficient K13.
FIG. 18 illustrates an antenna shape of a general NFC-compliant card. In the depicted antenna shape, a rectangular antenna coil is formed in a general IC-card shape of 85.6 mm×54.0 mm, which is defined by standards, such as ISO/IEC 7816-2 and JIS 6301-II, for use in FeliCa, RC-S860, and others, along an outer periphery of the card to ensure power as large as possible. Here, the ISO 14443 standard does not particularly define the shape of an antenna coil or the number of turns of the coil. It is recommended to form a coil so as to enclose a place where a contact IC card defined by the ISO/IEC 7816-2 standard contacts.
When the target is a no-power-supply card including an IC chip, power supply with a carrier may be performed in the above-described non-contact communication system simultaneously with data communication. The principle of operation of the non-contact communication system in this case is described with reference to FIGS. 19A to 19C.
FIG. 19A illustrates an equivalent circuit of two loop antennas for magnetic coupling with a flow of carriers between loop antennas at the time of data communication from an initiator to a target. The initiator performs ASK modulation on a carrier at 13.56 MHz sent from its own for data transfer to the target.
FIG. 19B illustrates a flow of carriers between loop antennas at the time of data communication from a target to an initiator and an equivalent circuit near a loop antenna on a target side. By changing its own antenna load with an electrical switch, the target performs modulation (load modulation) on non-modulated carriers at 13.56 MHz coming from the initiator for data transfer to the initiator.
FIG. 19C illustrates a flow of carriers between loop antennas at the time of power supply from an initiator to a target and an equivalent circuit near a loop antenna. The target rectifies carriers at 13.56 MHz sent from the initiator to obtain circuit driving power.
FIGS. 20A and 20B each schematically depict the state where an electromagnetic wave propagates from an antenna line. In general, an electromagnetic wave at a place sufficiently away from an output end of an antenna (half or longer than a wavelength λ of a carrier) propagates through the air with a magnetic field and an electric field in a pair. That is, when a current flows through an antenna line, a magnetic field first occurs, and then an electric field occurs in a direction perpendicular to this magnetic field. Then, a change is repeated alternately between a magnetic field and an electric field, proceeding as water ripples as depicted in these drawings. An electromagnetic wave having a wavelength λ equal to or greater than 0.1 mm (a frequency of 3000 GHz or lower) is called an electric wave.
As evident from FIGS. 20A and 20B, an electric wave radiated from a sufficiently distant place typically has a magnetic-field component. When strong electric waves associated with external wireless communication or the like are incident (refer to FIG. 21B) to a non-contact communication system using magnetic-field coupling (refer to FIG. 21A), communication is interfered and a trouble occurs. Such electric waves causing a communication trouble are referred to below as jamming waves.
When the communication characteristic of the non-contact communication system is sufficiently good, the influence of the jamming waves can be neglected. However, for example, when the distance between the antennas is long and high-speed communication degrades the characteristic, the influence of jamming waves becomes apparent.
In a non-contact communication system using 13.56 MHz in the past, the antenna has a strong frequency resonance characteristic with approximately 13.56 MHz as a peak to extend a communication distance and improve communication stability (refer to FIG. 22). Therefore, this system is hardly affected by jamming waves in a band of 90 to 800 MHz (TV broadcasting), a band of 800 MHz/1.5 GHz, a band of 2.0 GHz (portable phones), a band of 2.4 GHz/5 GHz (Bluetooth communication and wireless LAN), and others, used for many consumer wireless communications. Also, with a sufficiently low communication rate, the system can be sufficiently resistant to jamming waves at the time of reception.
By contrast, when the communication rate of the non-contact communication system is increased for large-capacity data transfer (described above), the frequency band of a transmission signal becomes wider proportionally. A wider signal frequency band means a flat frequency characteristic, thereby causing the system to be prone to be affected by disturbance. Therefore, for wide-band baseband communication, a mechanism of removing external jamming waves is typically desired.
As the most simple technique of improving the characteristic of wireless communication, a technique of increasing an output electric-wave intensity from a transmission side and improving an S/N ratio on a reception side can be taken. However, in the radio law enacted in each country, the electric-field intensity and the magnetic-field intensity that can be radiated to outside by a wireless device are restricted to prevent an adverse effect on other communication systems and the health of human body. The communication device for non-contact communication described above also abides by this law regulation when applied to commercial products.
FIG. 23 illustrates regulations for an output electric-wave intensity in a non-contact communication system (inductive read/write communication equipment) using 13.56 MHz stipulated in Ordinance for Enforcement of the Radio Law of Japan, Articles 44 and 46-2. Depending on the electric-field intensity discharged from the inductive read/write communication equipment, the application level necessary in the Ministry of Internal Affairs and Communications in Japan varies, roughly among the following three types (1) to (3).
(1) A communication device with its electric-field intensity within an extremely weak region depicted in FIG. 23 in all frequencies is an extremely-weak wireless station, and any application to the Ministry of Internal Affairs and Communications in Japan can be eliminated.
Here, an actually stipulated value according to the Radio Law is such that the electric-field intensity at a position 3 m away from the equipment is equal to or smaller than 500 μm/m. By contrast, in FIG. 23, for illustration in the same graph, values are converted to those at a position 10 m away from the equipment (150 μm/m). This stipulation is applied not only to conductive read/write communication equipment but also to wireless equipment using another band.
(2) When the stipulated value of the magnetic-field intensity in (1) above is not satisfied, a type-specific authentication can be made as long as the following four conditions are satisfied: a carrier frequency of 13.56 MHz; an error in carrier frequency is within 50 ppm; the electric-field intensity at a position 10 m away from equipment is within an individual authentication unnecessary region in FIG. 23 in all frequencies; and the entire spurious power is equal to or smaller than 50 μW. That is, a type specification of a communication device can be obtained through an application to the Ministry of Internal Affairs and Communication and, for those equivalent to the applied communication device (that is, for those identical in type to the applied communication device), any subsequent applications to the Ministry of Internal Affairs and Communications for equipment permission for each piece of equipment can be eliminated.
(3) When the condition (2) is not satisfied, an application is made to the Ministry of Internal Affairs and Communications for each piece of equipment to obtain equipment permission.
On the other hand, as for a magnetic-field intensity emitted from induction-type read/write communication equipment, the Radio Law stipulates that the amount of exposure for six minutes should be 0.16 mA or lower.
In general, when the electric-field intensity and the magnetic-field intensity of electric waves that can be output from the same loop antenna are compared, the electric-field intensity is restricted more severely by far (to a lower value). Therefore, in a non-contact communication system using loop-antenna magnetic-field coupling, output electric waves are restricted by a limit value of the electric-field intensity. This can be an obstacle to performance improvement of the non-contact communication system (such as extension of the communication distance and increase in speed).
In brief, the problems are as follows.
(1) Due to a steep frequency characteristic, the non-contact communication system using a 13.56 MHz band in the past is less prone to be affected by jamming waves. However, to increase an output electric-wave intensity with the aim of improving communication characteristics on a reception side, such as an S/N ratio, particular attention is paid to abiding by the restrictions on the electric-field intensity stipulated by the Radio Law.
(2) In a wideband baseband non-contact communication system, due to a flat frequency characteristic, the system is prone to be affected by jamming waves, and removal of disturbance is to be considered.
Japanese Unexamined Patent Application Publications Nos. 2004-153463, 2004-166176, and 2006-5836 are examples of related art.